LONDON (Reuters) - Former Russian double agent Sergei Skripal and his daughter Yulia remain critically ill in hospital after being poisoned with a nerve agent in the southern English city of Salisbury.

A still image taken from an undated video shows Sergei Skripal, a former colonel of Russia's GRU military intelligence service, being detained by secret service officers in an unknown location. RTR/via Reuters TV

This is what we know so far:

WHAT HAPPENED?

At 4:15 p.m. (1615 GMT) on March 4, Wiltshire police got a call from a member of the public about two people who were acting strangely.

Police found Skripal, 66, and Yulia, 33, unconscious on a bench outside The Maltings shopping centre in Salisbury.

The pair had no visible injuries and were taken to Salisbury District Hospital, where they are in critical condition.

Counter-terrorism police took control of the investigation on March 6. On Monday, Prime Minister Theresa May said the couple had been poisoned with a military-grade nerve agent and it was “highly likely” that Russia was responsible.

She gave Moscow until midnight on Tuesday to provide an explanation or warned it would face “extensive measures”.

WHAT POISONED THEM?

The Skripals were poisoned with a substance that was part of the Novichok group of nerve agents developed by the Soviet Union’s military in the 1970s and 80s, May told parliament.

Novichok agents are believed to be five to 10 times more lethal than more commonly known nerve agents VX and sarin poison gases.

In Salisbury, some police investigators wore chemical and biological protection suits and areas where the Skripals are known to have visited have been sealed off.

A police officer, Detective Sergeant Nick Bailey who was one of the first to respond to the incident, is in serious but stable condition in hospital where senior officers say he is making good progress.

British health officials have said there was only a limited risk to the public. However, hundreds of people who visited the city’s Zizzi restaurant or the Bishop’s Mill pub were told to wash their clothes after traces of the substance used to attack Skripal were found at both sites.

They were also advised to wipe down phones, handbags and other electronic items with baby wipes and wash items such as jewellery and spectacles with warm water and detergent.

WHAT IS A NERVE AGENT?

Nerve agents are small molecules based on phosphorus that interfere with nerve transmission, said Andrea Sella, professor of inorganic chemistry at University College London.

“In essence what they do is to block the mechanism that allows a nerve to reset itself after a signal has been transmitted,” he said. “This causes a pretty systemic collapse of many bodily functions.”

WHERE DID THE POISONING HAPPEN?

How the nerve agent was delivered remains the prime focus of the police inquiry, London’s Assistant Commissioner Neil Basu said.

Yulia Skripal arrived in Britain from Russia at London’s Heathrow Airport at about 2:40 p.m. (1440 GMT) on March 3.

At about 1:40 p.m. (1340 GMT) on March 4, the Skripals arrived at the car park of the Sainsbury’s supermarket store at The Maltings shopping centre.

Some time afterwards, they went to the Bishop’s Mill pub before going to Zizzi, an Italian restaurant, at about 2:20 p.m. (1420 GMT). They remained there until about 3:35 p.m. (1535 GMT). A member of the public alerted the emergency services at about 4:15 p.m. (1615 GMT), when they were found to be extremely ill.

Police have appealed for anyone who saw the Skripals or their car, a red BMW with the registration number HD09 WAO, between approximately 1 p.m. and 1.40 p.m. (1300 GMT and 1340 GMT).

WHO IS TO BLAME?

Hundreds of detectives are working on the investigation but police have said they would not declare a person of interest or any suspects while their enquiries were continuing.

However, May has pointed the finger of blame at Moscow and Foreign Secretary Boris Johnson has said it was “overwhelmingly likely” that the Russian state was involved.

They set a deadline of midnight on March 13 for the Kremlin to respond.

Russia says the allegations are false, provocative and has demanded that Britain hands over samples of the substance used to poison the Skripals.

U.S. President Donald Trump said he would talk to May and if he agreed with the British analysis that Russia was responsible “we will condemn Russia or whoever it may be”.

The European Union has said it would offer Britain support, while France and Germany are among British allies to have condemned the attack.

WHAT HAPPENS NOW?

If Russia fails to provide answers, May said she would discuss the British response at a meeting on Wednesday of her National Security Council. The proposed action will then be relayed to British parliament later.

Britain could call on allies for a coordinated Western response, freeze the assets of Russian business leaders and officials, expel diplomats, launch targeted cyber attacks and cut back participation in events such as the soccer World Cup.

The Russian Foreign Ministry said that British threats to target Moscow with punitive sanctions over the poisoning of a former spy would not be left unanswered.

Britain’s interior minister, Amber Rudd, has also asked police and the security services to look at allegations the Russian state was involved in 14 other deaths in Britain in recent years.

WHO IS SERGEI SKRIPAL?

Sergei Viktrovich Skripal was born in the Soviet Union on June 23, 1951.

Little is known about his early life but he served in the GRU military intelligence service. He reached the rank of colonel in the GRU but left in 1999 to work in the Russian foreign ministry. He also taught at the Russian Defence Ministry’s defence academy in Moscow.

Skripal was arrested in December 2004 by Federal Security Service agents on suspicion of treason: passing secrets to Britain’s MI6 intelligence agency. Skripal had been “turned” by MI6 in 1995, the FSB said, and had been in contact with British agents at the Moscow embassy.

In a secret trial at a Moscow military court, he confessed to his treachery and to selling the names, addresses and codenames of several dozen Russian agents to MI6, Russian media said when his conviction was announced in 2006.

Many of the Russian agents he betrayed were spies in Britain and Europe. Russian media said his motivation was financial: he received more than $100,000 paid into a Spanish bank account.

Russian state television said that Skripal had been recruited by the British when working as Russia’s military attache in Spain and that he had handed over 20,000 pages of secret documents to London.

Moscow’s respected Kommersant newspaper reported in 2006 that the FSB considered that the damage he had done to Russian spying operations was comparable to that caused by Oleg Penkovsky, another GRU colonel.

Penkovsky, a friend of the then GRU chief, informed British and American spies of a Moscow operation to place nuclear missiles in Cuba. The scandal led to the 1962 Cuban crisis and the world on the brink of nuclear war for several days.

Penkovsky was arrested in 1962 and executed in 1963 after being found guilty of high treason and espionage.

Skripal, who was shown wearing a track suit in a cage in court during his sentencing, was jailed for 13 years in 2006.

WHY IS SKRIPAL IN BRITAIN?

In June 2010, the U.S. Federal Bureau of Investigation said it had broken up an undercover Russian spy ring in the United States.

Dubbed the “illegals”, some of the Russians had spent years quietly collecting information and trying to meet Americans with political ties.

Skripal was pardoned by then-President Dmitry Medvedev and put on a plane for Vienna, where in July 2010 he was exchanged, along with three others, for the 10 Russian spies caught in the United States.

The spy swap, one of the biggest since the Cold War, took place on the tarmac of Vienna airport where a Russian and a U.S. jet parked side by side before the agents were exchanged.

One of the Russian spies exchanged for Skripal was Anna Chapman, who was feted as a hero by Moscow on her return.

WHY WAS HE IN SALISBURY?

Since emerging from the John le Carre world of high espionage and betrayal, Skripal lived modestly in Salisbury and kept out of the spotlight until he was found unconscious on Sunday.

His house in Salisbury was bought for 260,000 pounds ($360,000) in 2011. Skripal was listed at living there under his own name. In the years since he found refuge in Britain, Skripal lost both a wife and son.

His son Alexander died on July 18, 2017 aged 43 in St Petersburg, British media reported. The details are unclear.

His wife Liudmila died on Oct. 23, 2012 of cancer at the age of 60, according to British media. Reuters has not confirmed the exact cause of death of either.

WHAT IS GRU?

Russian military intelligence service is known by its Russian acronym GRU, which stands for Main Intelligence Directorate. Moscow’s other, better-known Foreign Intelligence Service (SVR) is the successor to the KGB’s First Chief Directorate.

Unlike the KGB, GRU was not split up when the Soviet Union collapsed. It has a special status and answers directly to the chief of the general staff, one of three people who control Russia’s portable nuclear trigger. GRU chiefs are picked by the president.

GRU has agents across the globe. It also has special forces units that fought in many post-World War Two conflicts including Afghanistan and Chechnya.

GRU, whose emblem features a bat hovering above the globe, was founded as the Registration Directorate in 1918 after the Bolshevik Revolution. Revolutionary leader Vladimir Lenin insisted on its independence from other secret services, which see GRU as a rival.

The public was given a rare chance to see parts of GRU’s Moscow headquarters when President Vladimir Putin visited it in 2006. He was shown taking part in shooting practice.